This invention relates to a fuel injection nozzle. More particularly, this invention relates to a fuel injection nozzle for a gas turbine combustion chamber with a film applicator provided with several fuel openings.
A great variety of methods is used to prepare the fuel-air mixture in gas turbine combustion chambers, with distinction being basically made between their application to stationary gas turbines or aircraft gas turbines and the respective specific requirements. However, in order to reduce pollutant emissions, in particular nitrogen oxide emissions, the fuel must generally be premixed with as much air as possible to obtain a lean combustion state, i.e. one characterized by air excess. Such a mixture is, however, problematic since it may affect stabilizing mechanisms in the combustion process.
FIG. 1 shows, in schematic sectional side view, a combustion chamber 10 and the corresponding fuel injection. Shown in the figure is a central supply of fuel in the burner axis 22 and a decentral supply of fuel 23 almost vertically to the burner axis. Arrowheads 11 and 12 schematically indicate the supply of air to an inner swirler 14 and to an outer swirler 15. The fuel-air mixture 13 enters the combustion chamber 10 in the usual manner.
Combustion is almost exclusively stabilized by the effect of swirling air, enabling the partly burnt gases to be re-circulated. Fuel is frequently introduced centrally by means of a nozzle arranged on the center axis of the atomizer. Here, fuel is in many cases injected into the airflow with considerable overpressure to achieve adequate penetration and to premix it with as much air as possible. These pressure atomizers are intended to break up the fuel directly. However, some designs of injection nozzles are intended to spray the fuel as completely as possible onto an atomizer lip. The fuel is accelerated on the atomizer lip by the airflow, broken up into fine droplets at the downstream end of this lip and mixed with air. Another possibility to apply the fuel onto this atomizer lip is by way of a so-called film applicator, in which case the fuel is distributed as uniformly as possible in the form of a film.
A further possibility to mix the fuel as intensely as possible with a great quantity of air is by decentral injection (FIG. 2) from the outer rim of a flow passage formed by a film applicator 1, which carries the major quantity of air. This can be accomplished from an atomizer lip, but also from the outer nozzle contour. Different to a film applicator, this type of injection is characterized by a defined penetration of the fuel into the main airflow.
Both, the injection of fuel by means of a central nozzle or a pressure atomizer and the introduction as a film by way of a film applicator are to be optimized such that a maximum amount of the air passing the atomizer, if possible the entire air, is homogeneously mixed with fuel prior to combustion. Characteristic of a low-pollutant, in particular low-nitrogen oxide combustion is the preparation of a lean fuel-air mixture, i.e. one premixed with air excess. However, this entails fuel nozzles whose flow areas are large enough to enable the high quantity of air to be premixed with fuel. Due to the size of these fuel nozzles and, if central injection is used, the limited ability of the fuel jets or sprays to penetrate the constantly increasing sizes of air passages and, thus, to provide a homogenous distribution of the fuel-air mixture, novel concepts of fuel injection and pre-mixture are required.
Homogenous distribution and introduction of fuel in large airflow passages calls for decentral injection from a maximum number of fuel openings to be arranged on the airflow passage walls. Due to their great number, however, the openings will be very small, as a result of which they may be blocked or clogged by contaminated fuel. Since these burners are frequently cut in at higher engine loads, blockage may also be caused by fuel degradation products if, after intermediate or high-load operation, burner operation via these fuel openings is deactivated and the fuel remaining in the fuel nozzle is heated up and degraded.
Typical of the fuel nozzles is, in many cases, a very irregular velocity and mass flow distribution in the radial direction. Due to the swirling air, which is required to stabilize the subsequent combustion process, the local airflows are at maximum in the area of the radially outer limiting wall. If fuel is introduced into the airflow via a small number of openings, the circumferential homogeneity of the fuel in the air will, on the one hand, be affected and, on the other hand, the fuel can penetrate very deeply into the flow and unintentionally mix and vaporize in regions in which air is not sufficiently available. This may also occur with decentral injection.